The use of plastics in medical engineering is often restricted due to inadequate surface properties. This concerns, in particular, the bio- and haemocompatibility of, e.g., vascular prostheses, catheters or sensors. Despite great endeavours up to the present day, vascular prostheses made of plastics with a diameter of &gt;6 mm may be used only on a restricted basis in view of the risk of thrombosis that nevertheless exists, and under no circumstances may vascular prostheses with a diameter of &lt;6 mm be used. The very large applications sector of artificial coronary vessels relates however, to a diameter of &lt;6 mm (see R. Zdrahala: Small Caliber Vascular Grafts, J. of Biomaterials Appl., Vol. 10, no. 4, Apr. 1996, p. 309-329).
Even when catheters are used, although these are usually only briefly in contact with the body, complications arise in many cases due to the lack of compatibility of plastics with the body, which can mean a series risk to the patient (see R. Dujardin, Polymerstruktur und Thrombogenitat, in Symposium Materialforschung-Neue Werkstoffe p. 724-749, PLR Jujlich 1994). The use of plastics is, however, unavoidable in many areas of medical engineering in view of their advantageous mechanical properties.
It is well known that materials based on titanium, niobium, tantalum, zirconium or hafnium have biocompatible properties (see Clarke E. G. and Hickman J., An investigation into the correlation between the electrical potential of metals and their behaviour in biological fluids, J. Bone Joint Surg., Vol. 35B (1953) 467). Moreover, the excellent haemocompatibility of titanium nitride was demonstrated (see Y. Mitamura, Development of a ceramic heart valve, in J. of Biomaterials Applications, Vol. 4 July 1989, p. 33-55). As yet, however, no material has existed which has both the advantageous mechanical properties of plastics and the outstanding compatibility properties of the compounds based on Ti, Ta, Nb, Zr and Hf.
It is well known that, as a result of chemical vapour deposition (CVD), it is also possible to coat substrates with a complicated geometry, of the kind used in medical engineering. However, in the case of the inorganic starting substances mostly used hitherto for coating, reaction temperatures of &gt;600.degree. C. are required with the materials under consideration (see S. Siveram, Chemical Vapor Deposition, Van Nostrand Reinhold, New York 1995). Moreover, it is well known that coating temperatures of about 300.degree. C. may be used as a result of using organometallic or metallo-organic starting substances (see Sugiyama: Low Temperature Deposition of Metal Nitrides by Thermal Decomposition of Organometallic Compounds, J. Electrochem. Soc., September 1975, p. 1545-1549). Moreover, it was reported that the coating temperatures may be reduced markedly by the use of lasers or by coupling a low-pressure plasma. The lowest known coating temperature of the materials in question is given as 150.degree. C. in connection with the deposition of Ti(C, N) and Zr (C, N) from Ti[N(CH.sub.3).sub.2 ].sub.4 and Zr[N(CH.sub.3).sub.2 ].sub.4 by plasma-activated CVD (PACVD), whereby a pulsed source of excitation of direct current was used to produce the plasma (see K -T. Rie et al., Studies on the synthesis of hard coatings by plasma-assisted CVD using metallo-organic compounds, Surface and Coating Technology 74-75 (1995), p. 362-368). This coating was applied to prevent wear on metallic substrates on which sufficient adhesion can be achieved without difficulty.
An implant for use in the human body is known from DE-A-195 06 188 which is composed of a substrate and a metal-containing coating. Although plastics are mentioned as substrate, metals are used as substrates in the examples. The coating may be applied both by CVD and PVD (physical vapour deposition).
The PVD process is preferred because it is pointed out in the introductory part of DE-A-195 01 188 that considerable heating of the substrate is required in the CVD process. As part of the teaching of DE-A-195 06 188, the CVD process is not, therefore, considered for coating plastics because these would be destroyed at the high temperatures mentioned. The PVD coatings described in DE-A-195 06 188 have substantial shortcomings:
It is very difficult and expensive (if not impossible) to coat complicated geometries such as e.g. textile implants (vascular prostheses, plastic nets for treating inguinal hernias etc. ) by PVD. The expert knows this phenomenon as the "shadowing effect". PA1 Roughening and/or an intermediate layer is usually required for sufficient adhesion (see, for example, claims 12 to 14 of DE-A-195 06 188). PA1 The layers are more or less porous, which is disadvantageous for corrosion resistance. Moreover, unwanted adsorption of, for example, blood constituents, may occur easily through the pores. PA1 No damage to the plastic substrate by the coating process, particularly by excessive thermal stress, PA1 Adequate adhesion of the layer, despite the extreme differences in the properties of the layer and the substrate, such as thermal expansion and elasticity, PA1 Smooth surface of the layer in order to minimise interactions, such as, e.g., adsorption of body fluids. PA1 M means one or more metals from the group comprising Ti, Ta, Nb, Zr and Hf, PA1 a=0.025 to 0.9 PA1 b=0.025 to 0.7 PA1 x=0.2to 0. 9 PA1 y=0 to 0.7 PA1 z=0 to 0.7 PA1 a+b+x+y+z=1 PA1 a) cleaning the plastic substrate, PA1 b) selecting a suitable metallo-organic or organometallic starting compound, PA1 c) converting the metallo-organic or organometallic starting compound, if it is not gaseous, to the gas phase, PA1 d) conveying the gaseous metallo-organic or organometallic compound starting compound by means of a carrier gas into a reactor in which is situated the plastic substrate, PA1 e) passing energy to the substrate by heating, plasma coupling, laser radiation, etc., such that a temperature of 100.degree. is not exceeded, and PA1 f) maintaining a pressure of 0.1 to 1030 mbar in the reaction chamber, and a pressure of &lt;50 mbar if a plasma is used. PA1 The adhesion is obtained by C--C bonds at the base of the layer and by the graduated build up of the coating, as a result of which a "smooth" transition from the plastic substrate to the ceramic layer is achieved. The presence of the C--C bonds at the base and of the M--C bonds at the surface of the layer may be determined by XPS analyses (XPS=Xray photoelectron spectroscopy). PA1 As a result of this build up, no roughening of the substrate is required to increase the adhesion. PA1 The layers are extremely smooth (R.sub.a =0.001 .mu.m) and continuous. As a result, the exchange surface is smaller than with rougher and porous layers. This leads to better corrosion resistance and bio- and haemocompatibility than with rougher and porous layers.
A process for the chemical vapour deposition of transition metal nitrides on various substrates is known from WO 91/08322. Preferred substrate temperatures are between 200 and 250.degree. C. In Example 6, polyester sheets which are kept at a temperature of 150.degree. C. and coated with titanium nitride are used as substrate. Ammonia must be used as reaction gas in this process. The addition of the extremely reactive ammonia unavoidably leads, however, to unwanted gas reactions (see S. Intemann, "Eigenschaften von CVD-Titanverbindungsschichten aus metallorganischen Verbindungen fur die Mehrlagenmetallisierung hochst integrierter Bauelemente" ("Properties of CVD titanium compound layers of metallo-organic compounds for multi-layer metallisation of highly integrated building components"), Dissertation TU Munich 1994), which in turn leads to an uneven layer build up and a very pronounced shadowing effect, with the result that the advantages of the CVD process over the PVD process are destroyed.
As yet, therefore, there is no known satisfactory process with which plastics may be coated in a satisfactory manner with metal-containing layers by CVD. Either the substrate temperatures required for coating are too high and lead to deterioration of the plastic substrate, or the known processes lead to coatings which are unsatisfactory. An overriding prejudice amongst experts was that composites cannot be produced from soft substrates and hard coating materials.
Thus, A. Bolz describes in his dissertation, Physikalische Mechanismen der Festkorper-Protein-Wechselwirkung an der Phasengrenze a-SiC--H-Fibrinogen [Physical mechanisms of solid-protein interaction at the a-SiC--H-fibrinogen interface], University of Erlangen-Nuremberg, 1991, that it is not possible to coat soft substrates such as, e.g., plastics, with hard coating materials such as, e.g., SiC on account of the "feather bed effect" which occurs.